We generate 2 one-loop, 12 two-loop, 206 three-loop and 5062 four-loop diagrams contributing to \(A_t\)  using QGRAF [1]}. We then generate FORM [2]} code for
each diagram using q2e [3]}, [4]}, compute the colour factors using COLOR [5]} and perform an expansion-by-subgraph [6]}
in the limit \(M_t^2 \gg s = M_h^2\)  using exp [3]}, [4]}.
As a result, all diagrams are mapped onto one- to four-loop massive tadpole integrals and one- to three-loop massless form factor integrals, separating the two scales \(M_t\)  and \(s\) .
